(adam smith)

I want to purchase and learn to use a lathe, because I need to make
new parts for some old 1930's air horns (diaphone fog horns) that are
not otherwise available. One part I need to make is the piston, which
looks like this:

http://www.airraidsirens.com/diaphon..._front_640.jpg
http://www.airraidsirens.com/diaphon..._close_640.jpg

It's 4" in diameter at the wide end, and 2.83" diameter over the rest,
by about 5" long overall. This piece fits concentrically inside a
brass cylinder, and it reciprocates at 180Hz when in use. The original
1918 shop drawings for this device spec the precision of the diameter
of the piston as "4 inches, to gauge limits--machine all over
outside". Newer drawings have the diameter as 4.000, +0.000/-0.002

It's interesting that this would have been cast aluminum back in 1918.

The cylinder is lathe finished, but the slots are probably made on a mill (a
horizontal mill, back then would have been more in use).

You really need to find some place where you can use the machinery, rather than
buying the tooling for making one/a few part(s).
Frank Morrison